1. Field of the Invention
The present invention relates to a tracking device installed in an apparatus for playing back a rotary recording medium, especially an apparatus of the type reproducing video signals and other data signals which are recorded on a rotary magnetic recording medium such as a magnetic disk.
2. Description of the Prior Art
A recent achievement in the realm of the imaging art is an electronic still camera system which uses in combination a solid state imaging device, an imaging tube or like image pick-up device, and a recording apparatus of the kind using a magnetic disk which is a relatively inexpensive and large-capacity recording medium. In an electronic still camera system, while a subject is picked up purely electronically by an image pick-up device in the form of a still picture to be recorded on a rotary disk by a recording apparatus, the picture is reproduced whenever desired by means of a television system, a printer or the like which is independent of the recording apparatus.
A rotary magnetic recording medium applicable to such an electronic still camera system, for example, may comprise a disk which has a diameter as small as about 50 millimeters and accommodates fifty concentric tracks at a track pitch of about 100 microns, i.e. with a track width of about 50 to 60 microns and a guard band width of about 50 to 40 microns. Loaded on a recording or a playback apparatus, the disk is rotated at a constant speed of, for example, 3,600 revolutions per minute so that a video signal is recorded on or reproduced out of the disk on a field or a frame basis.
A problem with a recording medium, especially a magnetic disk, which is applicable to magnetic recording as stated above is that tracking errors are apt to occur due to various causes such as compatibility, eccentricity and thermal expansion of the medium. Specifically, it often occurs that a magnetic playback head of a playback apparatus partially scans a track of the disk which is positioned next to a desired track, bringing about crosstalk between the nearby tracks.
To solve the above-described crosstalk problem, there has been proposed a system in which a magnetic record head is caused to record a tracking signal under the application of tracking servo, while tracking servo is applied to a magnetic playback head by utilizing the tracking signal. However, it is impractical for a tracking servo mechanism which needs utmost accuracy of control to be built in a camera and other recording instruments which are lightweight and of a compact size.
In light of the above situation, a recording system may be implemented with a guard band system or a frequency-modulation (FM) azimuth system. Such a recording system is effective to compensate for some degrees of tracking errors during playback by preventing a playback head from scanning a track other than and next to a desired one or, if it failed to do so, preventing it from picking up a signal from the nearby track.
Tracking control by envelope peak detection is another approach known in the art. In accordance with this kind of control, a record head is transported through each predetermined track pitch by a stepping motor without any tracking servo applied thereto. A playback head, on the other hand, is applied with tracking servo by detecting an envelope of a signal which is stored in each track and, based on a peak of the envelope, identifying an optimum track.
The head position at which the envelope shows a peak is determined by transporting a magnetic head over a predetermined pitch, then reading an envelope value at that position, and then comparing it with an envelope value which was detected at a previous head position.
For the comparison stated above, a digital processing system is advantageously used. In this condition, a video signal sensed by a magnetic playback head is subjected to envelope detection, and the resulting envelope output is fed to a digital processing system after being converted into a digital value by an analog-to-digital converter. Advantageously, only those envelope levels which exceed a predetermined threshold level are used for the comparison so that the detection may be freed from errors due to noise and other disturbances to the system.
The tracking control by envelope detection as discussed above in which tracking is performed by detecting a peak of an envelope at any desired or predetermined rotational phase angle of a magnetic disk still has a problem left unsolved. Specifically, should a magnetic disk loaded on a playback apparatus be eccentric, i.e., should the center of rotation of a magnetic disk be offset from the center of the circle of a track recorded, a playback head would not always remain on-track throughout the rotation of the disk and might become deviated from a position representative of the peak of the envelope and thus fail to reproduce information accurately from the track.